U.S. patent number 5,077,602 [Application Number 07/481,001] was granted by the patent office on 1991-12-31 for color difference compressor.
This patent grant is currently assigned to Eastman Kodak Company. Invention is credited to Gregory O. Moberg.
United States Patent |
5,077,602 |
Moberg |
December 31, 1991 |
Color difference compressor
Abstract
A color difference compressor operating in a color difference
baseband system compensates for illumination overload in the output
of a color sensor by switching a pair of multiplexers between the
active video portion of the color difference signals and a
predetermined blanking level. The output signal from the sensor is
separated into three colors by a sample/hold circuit prior to
generations of the color difference signals and a signal overload
in any one of the three colors triggers an overload control signal
that switches the multiplexers.
Inventors: |
Moberg; Gregory O. (Rochester,
NY) |
Assignee: |
Eastman Kodak Company
(Rochester, NY)
|
Family
ID: |
23910193 |
Appl.
No.: |
07/481,001 |
Filed: |
February 15, 1990 |
Current U.S.
Class: |
348/256; 348/674;
348/E9.053; 348/E9.01 |
Current CPC
Class: |
H04N
9/68 (20130101); H04N 9/0451 (20180801) |
Current International
Class: |
H04N
9/04 (20060101); H04N 9/68 (20060101); H04N
009/077 (); H04N 009/68 () |
Field of
Search: |
;358/27,21R,32,29C,35,170,172,174,213.16,29 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Chin; Tommy P.
Attorney, Agent or Firm: Woods; David M.
Claims
What is claimed is:
1. In a video signal processing circuit for compensating for
signals, generated by an image sensor, that have a non-linear
response to high illumination thereby causing respective portions
of the output signal to overload, the combination comprising:
means for generating a plurality of color signals from the output
signal of the image sensor;
means for detecting a signal overload in one or more of said color
signals;
means responsive to said detecting means for generating an overload
control signal;
means for simultaneously generating two baseband video signals from
the color signals, said baseband signals including a blanking
level;
means responsive to said overload control signal for identically
adjusting the level of both baseband video signals toward the
blanking level whenever a signal overload is detected in one or
more of said color signals.
2. A video signal processing circuit as claimed in claim 1 in which
said overload control signal generated by said means responsive to
said detecting means is a bi-level digital overload control signal
and said means responsive to said digital overload control signal
drives the baseband video signals to the blanking level whenever
the value of the digital overload control signal indicates said
overload.
3. A video signal processing circuit as claimed in claim 1 in which
said overload control signal generated by said means responsive to
said detecting means is an analog overload control signal and said
means responsive to said analog overload control signal drives the
baseband video signals toward the blanking level in proportion to
the level of the analog overload control signal.
4. In a video signal processing circuit for compensating for
signals, generated by an image sensor, that have a non-linear
response to high illumination thereby causing respective portions
of the output signal to overload, the combination comprising:
means for generating a plurality of color signals from the output
signal of the image sensor;
means for generating an overload signal whenever one or more of the
color signals exceeds a predetermined threshold voltage;
means for generating baseband video signals from the color signals,
said baseband signals including a blanking level;
means for providing a compression voltage having a level
corresponding to the blanking level;
multiplexing means having output means for providing the output
signal, first input means for receiving the baseband signals, and
second input means for receiving the compression voltage;
means responsive to the overload signal for switching the
compression voltage on said second input means to said output of
said multiplier means whenever one or more of the color signals
exceeds the predetermined threshold voltage, whereby the baseband
signal level is compressed to the blanking level when a signal
overload is detected.
5. A video signal processing circuit as claimed in claim 4 wherein
the baseband signals include a first color difference signal and a
second color difference signal, and said multiplexing means
includes a first multiplexer having first and second input means
for receiving the first color difference signal and the compression
voltage respectively, and a second multiplexer having first and
second input means for receiving the second color difference signal
and the compression voltage, respectively.
6. A video signal processing circuit as claimed in claim 4 wherein
the image sensor includes an array of picture elements, and said
switching means responsive to the overload signal includes a low
pass filter for receiving the overload signal and for providing a
filtered output signal for switching said multiplexer means, said
filtered output signal extending over a greater time period than
said overload signal whereby the effect of the compensation is
spread over several picture elements.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of video signal processing and,
more particularly, to the processing of color video signals to
compensate for the effect of sensor overload.
2. Description Relative to the Prior Art
Image sensors, particularly charge-coupled device (CCD) sensors,
are driven into a non-linear region of response when the photosites
are brightly illuminated and, in the case of CCD sensors, the
charge levels approach the maximum well capacity. This means that
the output signal from the sensor does not respond proportionately
to the input light. In such a condition, the sensor is said to be
overloaded, at least for the sensor photosites that experience such
a response. Color image sensors, of course, are sensitive to
several constituent colors which are combined in subsequent video
processing to form an output video signal representative of the
color of the input light. Furthermore, a given color is ordinarily
made up of unequal amounts of the constituent colors. Since,
therefore, for high illumination the charge residing in the
different color-sensitive photosites will be driven non-linear at
different points, the resultant color represented by the output
video signals shifts away from the desired color. Moreover, the
color shift is unpredictable. This produces visually unappealing
colors in local areas of the reproduced image.
It is known how to monitor the level of the color signals from the
sensor and to reduce a composite output video signal in some
controlled fashion when overload is detected. For example, Sony
Corporation has disclosed an NTSC processor circuit that provides
dynamic color compensation for an NTSC signal. The several color
signals from the sensor are compared to a threshold. When the
threshold is exceed by any one of the color signals, the modulated
chrominance signal is accordingly reduced or set to some
predetermined level. The result is to produce a white output for
the affected image points. It has been found that an ordinary
viewer tolerates a white error, rather than some arbitrary color,
and image quality is thereby preserved. It is further possible to
fade the whole NTSC signal under certain situations, such as when a
recording operation stops.
Color compensation systems operating in modulated chrominance space
cannot be conveniently adapted to other color systems. In
particular, it would be desirable to provide overload compensation
for systems producing baseband signals, such as systems producing
red, green, blue (RGB) or luminance and color difference signals
(Y, R-Y, B-Y).
SUMMARY OF THE INVENTION
Color compensation for sensor overload in a baseband video system
is provided by detecting a signal overload in one or more of the
separate color signals generated by a color sensor and accordingly
adjusting the level of the baseband signals toward the blanking
level when such an overload is detected. In one embodiment, a
circuit responsive to detection of an overload generates a bi-level
digital signal that is used to switch the baseband signal to the
blanking level, thus producing a white output. In another
embodiment, an analog control signal is used to drive the baseband
signals toward the blanking level in proportion to the level of the
analog control signal.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described in relation to the drawings, in
which
FIG. 1 is a block diagram of a color compressor circuit arranged
according to the invention;
FIGS 2A, 2B and 2C are waveform diagrams of several signals
appearing in the circuit of FIG. 1;
FIG. 3 is an alternative embodiment of the compression elements
shown in FIG. 1; and
FIGS. 4A and 4B are waveform diagrams of signals processed by the
alternative elements of FIG. 3.
DETAILED DESCRIPTION OF THE INVENTION
Referring first to FIG. 1, an imager 10 provides a stream of input
signals to a sample/hold circuit 12, which separates the input
color signals from the stream thereof and provides an output
constituting separate color signals. In the preferred embodiment
the imager 10 is a CCD imager chip manufactured by Sony Corp. (part
No. ICX022AK), which provides a resolution of 768 horizontal
photosites in 493 vertical lines, and the sample/hold circuit 12 is
a processor chip manufactured by Sony Corp. (part No.
CXA1337Q-Z/R). The particular sample/hold circuit employed in this
embodiment provides an output constituting separate green, cyan,
and yellow signals. Since the color sequence inherent in the output
signal from the imager 10 depends upon the particular color filter
array used with the sensor, the imager 10 and the sample/hold
circuit 12 must be accordingly timed pixel-by-pixel by a timing
circuit 14 to provide the requisite color separation. Other video
timing matters, such as composite sync, composite blanking, burst
flag, and the like, are provided by a composite timing circuit 16.
In the preferred embodiment, the timing circuit 14 is a IC chip
manufactured by Sony Corp. (part No. CXD1035BQ) which is adapted
for the color filter array pattern provided on the particular
imager chip 10 employed in this embodiment. Since the composite
timing circuit 16 is not critically related to the architecture and
pattern of either the imager 10 or the sample/hold circuit 12, it
may be provided by a conventional design.
The separated green,cyan, and yellow signals are applied to a color
processor 18, which utilizes color matrices or like algorithms to
mathematically process the input signals and to produce a baseband
luminance signal (Y) and two baseband color difference signals
(R-Y, B-Y). In the preferred embodiment the color processor 18 is
an IC chip manufactured by Sony Corp (part No. CXA1339Q-Z/R), which
is especially adapted to process the green, cyan, and yellow
signals provided by the sample/hold circuit 12. The output color
difference signals (R-Y, B-Y) are applied to respective clamps 20a,
20b for line-by-line clamping of the color difference signals (R-Y,
B-Y) to a dc black level provided by the clamp signal voltage. The
clamped signals (R-Y.sub.in, B-Y.sub.in) and a dc compression
voltage (V.sub.c) are applied to respective analog multiplexers
22a, 22b, which select between the two pairs of input signals and
provide a color compressed output constituted by the compressed
color difference signals (R-Y.sub.c, B-Y.sub.c)
The analog multiplexers 22a, 22b are switched to pass the
compression voltage (V.sub.c) whenever an overload is detected by a
thresholding circuit incorporated in the sample/hold circuit 12.
For instance, the three sampled outputs of the circuit 12 are
jointly connected to a comparator 24 for comparison with a
threshold voltage provided by a potentiometer 26. If any of the
color levels exceeds the threshold voltage, an overload control
signal is provided to a low pass filter 28. The filter 28 time
aligns the switching signal to the multiplexers 22a and 22b with
the color difference signals R-Y, B-Y, eliminates high frequency
white noise and noise spikes due to the sample/hold function, and
spreads the effect of color compression over a number of image
pixels, rather than the one or few pixels that have directly
experienced overload. The latter function improves the visual
aspect of overload by insuring that the compensation is spread over
several pixels. The low pass filter is a conventional fast attack,
slow decay type of filter design. The filtered output is applied to
a comparator 30, which functions as a 1-bit analog-to-digital
convertor and provides a digital overload signal (DIGITAL COMPRESS)
to the switching inputs of the multiplexers 22a and 22b.
The operation of the circuit of FIG. 1 can be further understood by
reference to the waveform diagrams of FIG. 2. The color difference
signal R-Y.sub.in that is applied to the analog multiplexer 22a is
shown in FIG. 2(A) to include a characteristic clamped blanking
portion and an active video portion. The problem of overload-caused
color shift arises during the active video portion when one or more
of the constituent color signals (the green, cyan, and yellow
signals, in this case) begin to respond non-linearly to impinging
light. The threshold potentimeter 26 is set accordingly to detect
this condition and the circuit 12 outputs an overload control
signal at such time to the low pass filter 28. The output of the
filter 28 is converted to the digital signal DIGITAL COMPRESS,
which is shown by FIG. 2(B) to extend (because of the low pass
filtering) over a range of image pixels. When the digital overload
signal DIGITAL COMPRESS goes high, the analog multiplexers 22a and
22b switch over to the compression voltage V.sub.c. In this
embodiment, the compression voltage V.sub.c is substantially the
same value as the clamp signal, that is, the same as the blanking
level of the color difference signal (R-Y.sub.in). The output
signal (R-Y.sub.c) shown in FIG. 2(C) is therefore compressed to
the blanking level for regions corresponding to the overload signal
(FIG. 2(B)). Such compressed regions correspond to white in the
reproduced image. Although not shown in FIG. 2, the other color
difference signal (B-Y.sub.in) is similarly processed.
FIG. 2 is also useful in illustrating that it is not always obvious
from the color difference signal where in the active video a
constituent color has overloaded. The overload is masked in the
color difference signal (but not in the reproduced image) in part
because the signal is a difference and in part because some low
amplitude output image colors may include one or more overloaded
constituents. For example, although flesh color is frequently of
relatively low amplitude, one of the constituent colors (green,
cyan, or yellow, in this system) may go non-linear and shift the
image color, often toward green. The color difference compressor
detects such situations and instead shifts the color toward white.
Overload compensation, therefore, is clearly a compromise but
nonetheless based on the proven assumption that shifts toward white
are visually more tolerable than uncontrolled, seemingly random,
shifts toward other colors.
The circuit illustrated thus far is a digital compensator that
essentially goes all the way to white for any overload. This was
preferred because it can be implemented economically with very few
parts. In many situations, analog compression will be preferable in
which the black level can be gradually approached in proportion to
the amount of overload. FIG. 3 illustrates circuit components that
can be substituted for the analog multiplexers 22a, 22b and the
comparator 30 to provide analog compression. The substitute
components include multipliers 40a, 40b for operating upon the
respective color difference signals (R-Y.sub.in, B-Y.sub.in). The
analog overload signal output from the filter 28 is inverted in the
negative gain amplifier 42 and clamped in clamp 44 to a maximum
voltage level (100% voltage). The effect is seen for a gradually
increasing overload signal in FIG. 4. The output signal (ANALOG
COMPRESS) from the low pass filter 28 is shown in FIG. 4(A) to
increase over time from 0% compression to 80% compression. The
inverted output signal (ANALOG COMPRESS.sub.in) after clamping is
shown in FIG. 4(B) to decrease over time from 100% voltage to 80%
voltage (relative to V.sub.DC). The latter signal, when applied to
the multipliers 40a and 40b brings the output signals (R-Y.sub.c,
B-Y.sub.c) down in unison from their uncompressed values to within
20% of the blanking level. The percentages chosen, of course, are
exemplary and the circuit can be driven to any level.
The foregoing disclosure is made in relation to certain commonly
available integrated circuit components. Other components, or a
custom design, can likewise be used in the practice of the
invention. In that case, the color separation provided by the
sample/hold function may deliver other colors, such as cyan,
yellow, magenta. The color processor 18 would then be changed to
accommodate such a different set of colors. Furthermore, the
generation of the overload control signal can be separately
accomplished (with, for example, the disclosed comparator and
potentiometer arrangement) rather than incorporated into the
sample/hold chip itself. Such modifications as these are within the
ordinary skill in this art.
The invention has been described in detail with particular
reference to a presently preferred embodiment, but it will be
understood that variations and modifications can be effected within
the spirit and scope of the invention.
* * * * *